IL21 and Parkinson disease: In our study, no significant correlations were found between the serum IL-21 concentrations and the number of cTfh in PD patients (P = 0.801 Figure 5A), however, we found that a positive trend of the correlation between the serum IL-4 concentrations and the number of cTfh in PD patients (P = 0.032, r = 0.353, Figure 5B).